Mega Camerupt, Mega Sharpedo and Mega Gallade revealed by CoroCoro

Pokémon Omega Ruby & Alpha Sapphire — 10 September, 2014

The latest issue of CoroCoro has leaked and it reveals three new Mega Evolutions and more details on Primal Kyogre and Primal Groudon.

First up is Mega Camerupt and Mega Sharpedo! They keep the same types (Fire/Ground and Water/Dark respectively) but have new abilities.
Mega Camerupt has Sheer Force. Its Speed decreases but other stats increase. It is used by Maxie, the Team Magma leader.
Mega Sharpedo has Strong Jaw. All its stats gain an increase. It is used by Archie, the Team Aqua leader.

Next is Mega Gallade! It stays Psychic/Fighting and has the ability Inner Focus.

Mega Gallade and Wally

Primal Kyogre's ability is translated as Ocean's Beginning and it activates Heavy Rain. It has a new move, translated as Ocean wave. Primal Groudon's ability is translated as Land's End and it activates Strong Sunlight. Its new move is translated as Cliff Blade.

It's currently unknown exactly what heavy rain and strong sunlight do. It could increase the number of turns of Drought/Drizzle, or make their effects more pronounced.

Rayquaza is also mentioned and stated to hold some secrets to Mega Evolution. No confirmation of whether it gets a Mega/Primal form though.

One final piece of news is that there will be a demo of OR/AS available for download from the eShop using a special code in a future CoroCoro. Pokemon used in the demo will be able to be transferred to the full game when released! No word yet on whether this will be available outside Japan.